This invention relates to an optical recording medium and more particularly, to a phase change optical recording medium configured to record and reproduce information thereon by irradiating laser light and having an excellent cross erasure property.
Optical recording mediums for recording and reproducing information by irradiating laser beams are large-capacity storage mediums satisfying the demands for a large capacity, quick accessibility and portability, which support the today's prosperity of personal computers. Among them, phase change optical recording mediums are recently being brought into practical use because of their simple recording principle. Explained below is an optical disc as an example of optical recording mediums.
The principle of recording and reproducing information on or from phase change optical recording mediums lies in irradiating semiconductor laser light onto a recording layer on a substrate and thereby reversibly change the status of a recording portion between amorphous and crystal phases. That is, upon recording data, a relatively high-power, short-pulse laser beam is irradiated onto a recording material to heat the recording portion to its melting point and thereafter rapidly cooling it to make an amorphous "recording mark". Upon reproducing data, a change in reflectance in the recording portion is read out as recording information. Upon erasing data, a lower-power, longer-pulse laser beam than the laser beam used during recording is irradiated onto the recording material to maintain that portion under a temperature not lower than the crystallizing temperature and lower than the melting point to crystallize it.
In this manner, when a phase change optical recording medium is used, it is sufficient to read out a change in reflectance between amorphous and crystalline states, and the optical system can be arranged easily. Additionally, phase change optical recording mediums have further advantages that they need no magnetic field which is inevitable in case of magneto-optical recording mediums, overwriting by optical intensity modulation is easy, and data transfer speed is high. Furthermore, they are well compatible with CD-ROM (compact disc-read only memory) and other existing discs exclusive for reproduction.
For improving the recording density of a phase change optical disc, there are various possible ways, such as decreasing the distance between recording marks, reducing the size of each recording mark, using a light source of a shorter wavelength, and others. As the method of decreasing the distance between recording marks, among those approaches, so-called "land groove (L/G) recording system and "mark length recording system", for example, have been proposed. The L/G recording system relies on recording data on both the "land track" "and the groove track" of an optical disc, and a high density approximately twice the conventional one can be expected with this system. The mark length recording system relies on detecting a change in reflectance in an edge portion of the recording mark (differential component of the reflectance), and a high density approximately 1.5 times the density of a conventional mark position recording method can be expected. If a super resolution technique proposed for ROM mediums is used in addition to these high-density recording techniques, it is expected that the current recording density of approximately 1.5 Gbpsi (bits/inch.sup.2) can be increased to 10 through 20 times.
Furthermore, it is considered effective for increasing the density to shorten the wavelength of the light source. In case of constricting a laser light source by using an optical lens, the minimum spot depends on the wavelength of the light source, and the light spot can be diminished as the wavelength becomes shorter. That is, the recording density can be increased in inverse proportion to the wavelength of the light source.
As reviewed above, phase change optical recording mediums have the possibility for a higher density. However, for enabling rewriting over or beyond tens thousands times, there still remain a lot of problems to be solved. That is, although the L/G recording system is an indispensable technique for increasing the track density, the problems of "cross erasure" and "cross talk" become noticeable as the inter-track distance becomes narrower. Both these problems are caused by the beam diameter being large relatively to the track width (diameter of the beam intensity e.sup.-2 is 0.92 .mu.m). Cross erasure is the phenomenon that a light beam irradiated onto a track to record data thereon undesirably erases a part of recorded marks in an adjacent track. This is a serious problem in destroying information to be maintained. Cross talk is the phenomenon that a light beam irradiated onto a track to reproduce information thereon undesirably reads out information on an adjacent track as well, and it directly leads to a reproduction error. Both these phenomena become noticeable together with the increase of track density, and must be solved preferentially to all for movements toward improvements of the recording density of recording mediums.